1. Field
This invention pertains to mining and ore leaching methods. In particular it pertains to a redox ore leaching method utilizing sulfurous acid to act either as an oxidizing or a reducing solution for heavy metals extraction.
2. State of the Art
Numerous ore leaching methods using sulfuric acid are known. As discussed in Wikipedia, Leaching (metallurgy) “Leaching” is a widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because no gaseous pollution occurs. There are a variety of leaching processes, usually classified by the types of reagents used in the operation. The reagents required depend on the ores or pretreated material to be processed. A typical feed for leaching is either oxide or sulfide.
For material in oxide form, a simple acid leaching reaction can be illustrated by the zinc oxide leaching reaction:ZnO+H2SO4→ZnSO4+H2O
In this reaction solid ZnO dissolves, forming soluble zinc sulfate.
In many cases other reagents are used to leach oxides. For example, in the metallurgy of aluminum, aluminum oxide is subject to leaching by alkali solutions:Al2O3+3H2O+2NaOH→2NaAl(OH)4 
Leaching of sulfides is a more complex process due to the refractory nature of sulfide ores. It often involves the use of pressurized vessels, called autoclaves. A good example of the autoclave leach process can be found in the metallurgy of zinc. It is best described by the following chemical reaction:2ZnS+O2+2H2SO4→2ZnSO4+2H2O+2S
This reaction proceeds at temperatures above the boiling point of water, thus creating a vapor pressure inside the vessel. Oxygen is injected under pressure, making the total pressure in the autoclave more than 0.6 MPa.
The leaching of precious metals such as gold can be carried out with cyanide or ozone under mild conditions
Heap leaching is an industrial mining process to extract precious metals, copper, uranium, and other compounds from ore. The process has ancient origins; one of the classical methods for the manufacture of copperas (iron sulfate) was to heap up iron pyrite and collect the leachate from the heap, which was then boiled with iron to produce iron sulfate.
The mined ore is usually crushed into small chunks and heaped on an impermeable plastic and/or clay lined leach pad where it can be irrigated with a leach solution to dissolve the valuable metals. While sprinklers are occasionally used for irrigation, more often operations use drip irrigation to minimize evaporation, provide more uniform distribution of the leach solution, and avoid damaging the exposed mineral. The solution then percolates through the heap and leaches both the target and other minerals. This process, called the “leach cycle,” generally takes from one or two months for simple oxide ores (e.g., most gold ores) to two years (for nickel laterite ores). The leach solution containing the dissolved minerals is then collected, treated in a process plant to recover the target mineral and in some cases precipitate other minerals, and then recycled to the heap after reagent levels are adjusted. Ultimate recovery of the target mineral can range from 30% of contained (run-of-mine dump leaching sulfide copper ores) to over 90% for the easiest to leach ores (some oxide gold ores).
The crushed ore is irrigated with a dilute alkaline cyanide solution. The solution containing the dissolved precious metals (“pregnant solution”) continues percolating through the crushed ore until it reaches the liner at the bottom of the heap where it drains into a storage (pregnant solution) pond. After separating the precious metals from the pregnant solution, the dilute cyanide solution (now called “barren solution”) is normally re-used in the heap-leach-process or occasionally sent to an industrial water treatment facility where the residual cyanide is treated and residual metals are removed. In very high rainfall areas, such as the tropics, in some cases there is surplus water that is then discharged to the environment, after treatment, posing possible water pollution if treatment is not properly carried out.
For Copper Ores, sulfuric acid is used to dissolve copper from its ores. The acid is recycled from the solvent extraction circuit (see solvent extraction-electrowinning, SX/EW) and reused on the leach pad. A byproduct is iron (II) sulfate, jarosite, which is produced as a byproduct of leaching pyrite, and sometimes even the same sulfuric acid that is needed for the process. Both oxide and sulfide ores can be leached, though the leach cycles are much different and sulfide leaching requires a bacterial or “bio-leach” component. The largest copper heap leach operations are in Chile, Peru, and the southwestern United States.
Although the heap leaching is a low cost-process, it normally has recovery rates of 60-70%, although there are exceptions. It is normally most profitable with low-grade ores. Higher-grade ores are usually put through more complex milling processes where higher recoveries justify the extra cost. The process chosen depends on the properties of the ore.
For nickel ores, the method is an acid heap leaching method like that of the copper method in that it utilizes sulfuric acid instead of cyanide solution to dissolve the target minerals from crushed ore. The amount of sulfuric acid required is much higher than for copper ores (as high as 1,000 kg of acid per tonne of ore, but 500 kg is more common.).
Nickel recovery from the leach solutions is much more complex than for copper and requires various stages of iron and magnesium removal, and the process produces both leached ore residue (“ripios”) and chemical precipitates from the recovery plant (principally iron oxide residues, magnesium sulfate and calcium sulfate) in roughly equal proportions. Thus, a unique feature of nickel heap leaching is the need for a tailings disposal area.
The method for uranium ores is similar to copper oxide heap leaching, also using dilute sulfuric acid. The final product is yellowcake and requires significant further processing to produce fuel-grade feed.
Acid leaching, according to the April 2004, v. 58, no. 2; p. 343-351 article by P. K. Abraitis et al. entitled “Acid Leaching and Dissolution of Major Sulphide Ore Minerals: processes and galvanic effects in complex systems is also affected by the electrical potential of the recovery mechanisms.
The sulphide ore minerals sphalerite, galena and chalcopyrite provide the major sources of the world's base metals (Zn, Pb, Cu), whereas pyrite is virtually ubiquitous as a metalliferous mineral in sulphide ore deposits. Acid leaching of these minerals occurs in nature in the context of acid mine drainage (AMD) and acid rock drainage and is of great importance in metal extraction using hydrometallurgical methods, including dump leaching of low-grade ores.
The rates and mechanisms of acid leaching of sulphide minerals of metal sulphides in mixed-mineral systems can be dramatically affected by galvanic effects, with rates increasing by factors as great as ˜30 times in some cases. The combination of conventional bulk leaching experiments with surface analysis techniques can lead to new insights into the mechanisms of the dissolution process through an understanding of reaction stoichiometry.
Pesic, U.S. Pat. No. 4,816,235 issued Mar. 28, 1989 discloses a method for obtaining silver and manganese metal from a silver-manganese ore including the step of leaching the ore with acidified thiourea. Reddin et al. U.S. Pat. No. 5,534,234 issued Jul. 9, 1996 discloses a method of recovering manganese in the form of manganese carbonate from ores containing manganese and iron contained in sulfurous acid leach solutions in a divalent state. Hunter et al., U.S. Pat. No. 5,914,441 issued Jun. 22, 1999 discloses a bioleaching method and apparatus for anaerobic oxidation of metal sulfides in ores and concentrates using sulfur-oxidizing bacteria, such as Thiobacillus ferrooxidans, Thobacillus thiooxidans, or Sulfolobus sp.
Cited for general interest are: Harmon et al, U.S. Pat. No. 7,566,400 issued Jul. 28, 2009 discloses a wastewater chemical/biological treatment method and apparatus for saline wastewater treatment generating biofuels. Harmon et al, U.S. Pat. No. 7,455,773 issued Nov. 25, 2008 discloses a package wastewater chemical/biological treatment plant recovery apparatus and method including soil SAR conditioning. Theodore, U.S. Pat. No. 7,416,668 issued Aug. 26, 2008 discloses a wastewater chemical/biological treatment plant recovery apparatus and method employing sulfurous acid disinfection of wastewater for subsequent biological treatment. Theodore, U.S. Pat. No. 7,563,372 issued Jul. 21, 2009 discloses a package dewatering wastewater treatment system and method including chemical/mechanical separation via drain bags and metal hydroxide removal via lime precipitation. Theodore, U.S. Pat. No. 7,429,329 issued Sep. 30, 2008 discloses a hybrid chemical/mechanical dewatering method and apparatus for sewage treatment plants employing sulfurous acid and alkalinization chemical treatment along with mechanical separation. Theodore et al, U.S. Pat. No. 7,967,990 issued Jun. 28, 2011 discloses a hybrid chemical/mechanical dewatering method for inactivating and removing pharmaceuticals and other contaminants from wastewater employing a sulfurous acid and lime acidification/alkalinization cycle, and an oxidation/reduction cycle to selectively precipitate, inactivate, and remove pharmaceuticals from wastewater. Gong et al, U.S. Pat. No. 7,967,989 issued Jun. 28, 2011 discloses a groundwater recharging wastewater disposal method and apparatus using sulfurous acid acidification to enhance soil aquifer treatment. Harmon et al, U.S. Pat. No. 7,867,398 issued Jan. 11, 2011 discloses a method and apparatus to reduce wastewater treatment plant footprints and costs by employing vacuum recovery of surplus sulfur dioxide. The above methods all use sulfurous acid and are therefore dependent upon the sulfur dioxide, sulfite, and bisulfite concentrations in solution and the oxidation/reduction potential of a desired reaction.
Sulfurous acid behaves as both an oxidizing and reducing agent, see J. Am. Chem. Soc., 1929, 51 (5) pp 1409-1428, “The Potential of Inert Electrodes in Solutions of Sulfurous Acid and Its Behavior as an Oxidizing and Reducing Agent” by Arthur A. Noyes, Harold H. Steinour. Consequently, where the ores to be treated vary in metal composition and valence states, alkaline and mixed mineral states, or require biological treatment requiring either a reducing agent or oxidizing agent, there remains a need for a leaching method to regulate the electrical reduction potential of sulfurous acid leaching solutions. The method described below provides such a pre-treatment method.